1. Field of the Invention
The present invention relates to an implantable medical device and to a method for assembling an implantable medical device. It further relates to a pre-fabricated header and to a hermetically sealed housing for use in an implantable medical device.
The present invention is applicable to a variety of implantable medical devices of the type having a pre-fabricated part to be coupled to a housing part. Examples of such devices are heart stimulators such as cardiac pacemakers and implantable cardioverter-defibrillators (ICD's). However, the invention and its background will be described herein in terms of a specific example of such implantable devices, namely cardiac pacemakers.
2. Description of the Prior Art
Present day cardiac pacemakers are typically designed to be implanted in a “pocket” of fatty tissue near the patient's upper chest or lower abdomen. Accordingly, electronic circuits within the pacemaker are hermetically sealed within a housing made of a material that is compatible with body tissue. Electrical connection from the outside with the electronic circuits within the hermetically sealed housing is accomplished via a connector assembly, often referred to as a header or header assembly, that is arranged on the pacemaker housing. Feedthrough terminals that pass through the hermetically sealed housing are connected with the pacemaker electronic circuits in the housing and with a lead pin receptacle in the connector assembly. At a proximal end, a lead conductor is provided with a lead pin for being received in the connector receptacle and at a distal end, the lead conductor is provided with electrodes for electrical tissue stimulation at the desired tissue location.
In order to ensure good electrical contact between the circuits within the housing and the outside, it is desirable to achieve a strong and reliable joint between the header and the housing.
Traditionally, a pacemaker is assembled by positioning the components of the header in connection with the hermetically sealed housing in a mold, adding a molding compound and molding the header on top of the hermetically sealed housing. A disadvantage with this known method is that it is complex and time consuming.
According to another known pacemaker design, as for instance disclosed in U.S. Pat. No. 4,182,345, the header is pre-molded. On the housing, there are projecting posts provided with through holes. In the header, complementary holes for receiving the posts and through going holes are provided. The through going holes of the header are arranged such that they aligned with the through going holes of the posts when the header is mounted on the housing. The header is locked to the housing by cross pins passing through the aligned through going holes of the post and the header. A disadvantage with this known device is that only very small tolerances are acceptable and that, in view of the size of the components, the mounting operation will be complicated and difficult.
In United States Patent Application Publication No. 2004/0093038 a pre-molded header assembly for an implantable medical device is disclosed. The bottom wall of the header is supported by a support plate of metal and retained in place in the header by a peripheral undercut. The housing of the implantable medical device has a planar upper surface. The pre-molded header can be mounted to the medical device by welding the support plate to the upper surface of the housing. A problem with this known device is that, the welding operation may cause a severe heat development unless it is very carefully performed. As a consequence, the polymeric material of the header may melt and the conductor arrangements of the header may be damaged. Furthermore, the heat may cause a leakage in the hermetically sealed housing. If the welding operation fails, both the header and the housing must be discarded.